Any flu virus can be dangerous for a pregnant woman, but swine flu appears to be especially threatening.

According to data from the Centers for Disease Control and Prevention, 15 out of 266 swine flu deaths in the U.S. so far have been among pregnant women. That's 6 percent of total fatalities, even though pregnant women only make up 1 percent of the population.

The flu can be particularly deadly in the second and third trimester of pregnancy, both for mother and baby.

Today, Dr. Jacques Moritz, director of gynecology at New York's St.Luke's Roosevelt Hospital, said that's because a pregnant woman's weakened immune system makes her especially susceptible to the virus.

"What the body does, and what the baby does, really, is it sends out a message, 'Don't get rid of me,'" Moritz said. "So it brings down the immune system, so you don't reject the fetus."

He added, "Anybody that is going to be pregnant, in the second or third trimester during this upcoming flu season, should get vaccinated before, obviously, the flu season."

Pregnant Women Have Already Battled Swine Flu

The first American to die of swine flu was a 33-year-old pregnant woman in Texas. Her baby girl, delivered by caesarian, survived.

Near Seattle, 27-year-old Katie Flyte is fighting for her life after complications from swine flu. Flyte was six months pregnant and caring for her sick 2-year-old when she developed flulike symptoms that were misdiagnosed. She developed pneumonia, then respiratory failure, as doctors rushed to save her premature daughter.

"I've got to give my baby girl a name without a mom to help me make the decision," Katie's husband, Kenny Flyte, told KOMO News last week.

Meantime, in Australia, doctors are urging pregnant women to stay home during this swine flu season to avoid contact with anyone suffering from the illness. In Great Britain, this fall, there may be advice to pregnant women is to avoid large social gatherings.

Today Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, said it's "understandable" that pregnant women are hesitant to get vaccinated.

"That's a natural reflex, but you have to understand the balance of the risk of a vaccination," Fauci added.

Given his heartbreaking experience, Kenny Flyte advises women who are expecting to act quickly if they feel sick.

"Go see your doctor," Flyte said. "Don't even play around."

Clinical Trials for Vaccine to Start in August

Fauci said today that health officials "appear to be having no trouble getting people signing up as volunteers" for clinical trials expected to begin in August to test the vaccine.

The clinical trials for a vaccination are expected to be followed by a voluntary vaccine program, anticipated to begin mid-October.

That vaccine program would be limited at first to those most susceptible to the virus. Those first in line to receive the series of two shots would likely be children, teenagers, health workers and pregnant women. The vaccine would be purchased by the federal government and distributed to state and local health departments.

At that time, Health and Human Services Secretary Kathleen Sebelius called on all states to update their emergency plans, including ways to carry out a mass vaccination program.

"We want to start re-engaging the American public and our state and local and health and private business partners in making sure we use these summer months well to prepare for what could be a serious outbreak," Sebelius said. "If it doesn't happen, we'll be fortunate and this planning won't go to waste."